The present invention relates to an image processing system applied to an image processing apparatus, for example, a digital copying machine to which an image of an original read by means of a scanner is input, and which subjects the input image to predetermined image processes, such as a density adjusting process, and outputs the processed image onto a paper sheet by means of an electrophotography laser printer.
Generally, in an image processing apparatus such as a digital copying machine for treating image data, image data is read from an original by image reading means, e.g., a scanner, and digitized. The digital image data is processed in accordance with the purpose, multileveled, and output through image output means, e.g., a laser printer. At this time, even if the image data read by the scanner includes a background portion of the original or the letters in the original are light-colored, the density of the print can be automatically adjusted so as to be suited for the original by a density adjusting function without manual adjustment using a density adjustment button.
Recently, as the density adjusting function, a method is developed, in which a density histogram is made on the basis of the input image data, the kind of the input original (whether the background is light or dark) is determined on the basis of the characteristic value of the density histogram, a reference value is obtained from the result of the determination, and the density of the input image data is adjusted in accordance with the reference value.
A reference value means a value used for automatically adjusting the density of an image by means of the characteristic value of an original. A white reference value is obtained for adjusting the density of a background and a black reference value is obtained for adjusting the density of a letter. An image density is adjusted by means of the two reference values. However, if the two reference values are very close, the density of lightly printed letters may be uneven, or a dark background cannot be removed and may remain like noise in places. In this case, the image quality may often be deteriorated.
Further, since the range for determining a white reference value cannot overlap the range for determining a black reference value, the dark background cannot be completely removed or a light-colored letter cannot be darkened.
Accordingly, an object of the present invention is to provide an image processing method, by which automatic adjustment for removing a background or darkening a light letter can be performed suitably without failure.
In order to achieve the above object, according to one aspect of the present invention, there is provided an image processing method comprising the steps of:
preparing a density histogram from input image data;
calculating a first white reference value and a first black reference value for adjusting density of the input image data based on the density histogram;
providing a final white reference value and a final black reference value by correcting at least one of the first white reference value and the first black reference value such that the first black reference value is at least a predetermined value greater than the first white reference value; and
adjusting the density of the input image data based on the final white reference value and the final black reference value.
The calculating step includes: calculating a second white reference value by giving a first offset to the first white reference value; and calculating a second black reference value by giving a second offset to the first black reference value, and
the providing step includes: comparing the second white reference value with the second black reference value; and providing the final black reference value by adding a predetermined density value to the second white reference value and providing the second white reference value as the final white reference value, if, as a result of the comparing, the second black reference value is smaller than a sum of the second white reference value and a constant value.
The calculating step includes: calculating as the first white reference value a density having a maximum frequency in a first predetermined density range for determining a white reference value; and calculating as the first black reference value a density having a maximum frequency in a second predetermined density range for determining a black reference value, wherein the first and the second predetermined density ranges partially overlap.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.